1、Designation: E385 11Standard Test Method forOxygen Content Using a 14-MeV Neutron Activation andDirect-Counting Technique1This standard is issued under the fixed designation E385; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the
2、 year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the measurement of oxygenconcentration in almost any matrix by using a 14-MeV neutron
3、activation and direct-counting technique. Essentially, the samesystem may be used to determine oxygen concentrationsranging from under 10 g/g to over 500 mg/g, depending onthe sample size and available 14-MeV neutron fluence rates.NOTE 1The range of analysis may be extended by using higherneutron fl
4、uence rates, larger samples, and higher counting efficiencydetectors.1.2 This test method may be used on either solid or liquidsamples, provided that they can be made to conform in size,shape, and macroscopic density during irradiation and countingto a standard sample of known oxygen content. Severa
5、lvariants of this method have been described in the technicalliterature. A monograph is available which provides a compre-hensive description of the principles of activation analysisusing a neutron generator (1).21.3 The values stated in SI units are to be regarded asstandard. No other units of meas
6、urement are included in thisstandard.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory lim
7、itations prior to use. Specific precau-tions are given in Section 8.2. Referenced Documents2.1 ASTM Standards:3E170 Terminology Relating to Radiation Measurements andDosimetryE181 Test Methods for Detector Calibration andAnalysis ofRadionuclidesE496 Test Method for Measuring Neutron Fluence andAvera
8、ge Energy from3H(d,n)4He Neutron Generators byRadioactivation Techniques2.2 U.S. Government Document:Code of Federal Regulations, Title 10, Part 2043. Terminology3.1 Definitions (see also Terminology E170):3.1.1 acceleratora machine that ionizes a gas and electri-cally accelerates the ions onto a ta
9、rget. The accelerator may bebased on the Cockroft-Walton, Van de Graaff, or other designtypes (1). Compact sealed-tube, mixed deuterium and tritiumgas, Cockcroft-Walton neutron generators are most commonlyused for 14-MeV neutron activation analysis. However,“pumped” drift-tube accelerators that use
10、replaceable tritium-containing targets are also still in use. Reviews of operationalcharacteristics, descriptions of accessory instrumentation, andapplications of accelerators used as fast neutron generators foractivation analysis are available (2,3).3.1.2 comparator standarda reference standard of
11、knownoxygen content whose specific counting rate (counts min1mgof oxygen1) may be used to quantify the oxygen content of asample irradiated and counted under the same conditions.Often, a comparator standard is selected to have a matrixcomposition, physical size, density and shape very similar tothe
12、corresponding parameters of the sample to be analyzed.3.1.3 14-MeV neutron fluence ratethe areal density ofneutrons passing through a sample, measured in terms ofneutrons cm2s1, that is produced by the fusion reaction ofdeuterium and tritium ions accelerated to energies of typically150 to 200 keV in
13、 a small accelerator. Fluence rate has beencommonly referred to as “flux density.” The total neutronfluence is the fluence rate integrated over time.1This test method is under the jurisdiction ofASTM Committee E10 on NuclearTechnology and Applications and is the direct responsibility of Subcommittee
14、E10.05 on Nuclear Radiation Metrology.Current edition approved Nov. 1, 2011. Published November 2011. Originallyapproved in 1969. Last previous edition approved in 2007 as E385 07. DOI:10.1520/E0385-11.2The boldface numbers in parentheses refer to a list of references at the end ofthe text.3For refe
15、renced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4Available from the Superintendent of Documents, U.S. Government Print
16、ingOffice, Washington, DC 20402.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.3.1 DiscussionThe3H(d,n)4He reaction is used to pro-duce approximately 14.7-MeV neutrons. This reaction has aQ-value of + 17.586 MeV.3.1.4 monitorany
17、 type of detector or comparison refer-ence material that can be used to produce a response propor-tional to the 14-MeV neutron fluence rate in the irradiationposition, or to the radionuclide decay events recorded by thesample detector. A plastic pellet with a relatively high oxygencontent is often u
18、sed as a monitor reference in dual sampletransfer systems. It is never removed from the system regard-less of the characteristics of the sample to be analyzed. It isimportant to distinguish that the monitor, whether an indepen-dent detector or an activated reference material, is not astandard used t
19、o scale the oxygen content of the samples to bemeasured, but rather is used to normalize the analysis systemamong successive analytial passes within the procedure.3.1.5 multichannel pulse-height analyzeran instrumentthat receives, counts, separates, and stores, as a function oftheir energy, pulses f
20、rom a scintillation or semi-conductorgamma-ray detector and amplifier. In the 14-MeV instrumentalneutron activation analysis (INAA) determination of oxygen,the multichannel analyzer may also be used to receive andrecord both the BF3neutron detector monitor counts and thesample gamma-ray detector cou
21、nts as a function of steppedtime increments (4-6). In the latter case, operation of theanalyzer in the multichannel scaler (MCS) mode, an electronicgating circuit is used to select only gamma rays within theenergy range of interest.3.1.6 transfer systema system, normally pneumatic, usedto transport
22、the sample from an injection port (sometimesconnected to an automatic sample changer) to the irradiationstation, and then to the counting station where the activity ofthe sample is measured. The system may include componentsto ensure uniform positioning of the sample at the irradiationand counting s
23、tations.4. Summary of Test Method4.1 The weighed sample to be analyzed is placed in acontainer for automatic transfer from a sample-loading port tothe 14-MeV neutron irradiation position of a particle accelera-tor. After irradiation for a pre-selected time, the sample isautomatically returned to the
24、 counting area. A gamma-raydetector measures the high-energy gamma radiation from theradioactive decay of the16N produced by the (n,p) nuclearreaction on16O. The number of counts in a pre-selectedcounting interval is recorded by a gated scaler, or by amultichannel analyzer operating in either the pu
25、lse-height, orgated multiscaler modes. The number of events recorded forsamples and monitor reference standard are corrected forbackground and normalized to identical irradiation and count-ing conditions. If the sample and a monitor reference sampleare not irradiated simultaneously, the neutron dose
26、 receivedduring each irradiation must be recorded, typically by use of aBF3neutron proportional counter. The amount of total oxygen(all chemical forms) in the sample is proportional to thecorrected and normalized sample count and is quantified by useof the corrected and normalized specific activity
27、of the com-parator standard(s).4.1.116N decays with a half-life of 7.13 s by b-emission (7),thus returning to16O. Sixty seven percent of the decays areaccompanied by 6.12863-MeVgamma rays, 4.9 % by 7.11515-MeV gamma rays, and 0.82 % by 2.7415-MeV gamma rays.Other lower intensity gamma rays are also
28、observed. About28 % of the beta transitions are directly to the ground stateof16O. (All gamma-ray energies and decay schemes are givenin Ref (8). Useful elemental data including calculated sensi-tivities and reaction cross-sections for (14-MeV INAA) areprovided in Refs (3) and (9). (See also Test Me
29、thods E181.)5. Significance and Use5.1 The conventional determination of oxygen content inliquid or solid samples is a relatively difficult chemicalprocedure. It is slow and usually of limited sensitivity. The14-MeV neutron activation and direct counting techniqueprovides a rapid, highly sensitive,
30、nondestructive procedure foroxygen determination in a wide range of matrices. This testmethod is independent of the chemical form of the oxygen.5.2 This test method can be used for quality and processcontrol in the metals, coal, and petroleum industries, and forresearch purposes in a broad spectrum
31、of applications.6. Interferences6.1 Because of the high energy of the gamma rays emittedin the decay of16N, there are very few elements that willproduce interfering radiations; nevertheless, caution should beexercised.19F, for example, will undergo an (n,a) reaction toproduce16N, the same indicator
32、radionuclide produced fromoxygen. Because the cross section for the19F(n,a)16N reactionis approximately one-half that of the16O(n,p)16N reaction, acorrection must be made if fluorine is present in an amountcomparable to the statistical uncertainty in the oxygen deter-mination. Another possible inter
33、fering reaction may arise fromthe presence of boron.11B will undergo an (n,p) reaction toproduce11Be. This isotope decays with a half-life of 13.81 s,and emits several high-energy gamma rays with energies in therange of 4.67 to 7.98 MeV. In addition, there is Bremsstrahlungradiation produced by the
34、high energy beta particles emittedby11Be. These radiations can interfere with the oxygen deter-mination if the oxygen content does not exceed 1 % of theboron present.6.2 Another possible elemental interference can arise fromthe presence of fissionable materials such as thorium, uranium,and plutonium
35、. Many short-lived fission products emit high-energy gamma rays capable of interfering with those from16N.NOTE 2Argon produces an interferent,40Cl, by the40Ar(n,p)40Cl re-action. Therefore, argon should not be used for the inert atmosphereduring sample preparation for oxygen analysis.40Cl (t12 = 1.3
36、5 m) hasseveral high-energy gamma rays, including one at 5.8796 MeV with ayield of 4.1 %.6.3 An important aspect of this analysis that must becontrolled is the geometry during both irradiation and count-ing. The neutron source is usually a disk source. Hence, thefluence rate decreases as the inverse
37、 square at points distantfrom the target, and less rapidly close to the target. Because ofthese fluence rate gradients, the irradiation geometry should bereproduced as accurately as possible. Similarly, the positioningof the sample at the detector is critical and must be accuratelyE385 112reproducib
38、le. For example, if the sample is considered to be apoint source located 6 mm from a cylindrical sodium iodide(NaI) detector, a 1-mm change in position of the sample alongthe detector axis was found to result in a 3.5 to 5 % change indetector efficiency (10). Since efficiency is defined as thefracti
39、on of gamma rays emitted from the source that interactwith the detector, it is evident that a change in efficiency wouldresult in an equal percentage change in measured activity andin apparent oxygen content. The sample and monitor (ifpresent) may be rotated during exposure or counting, or both,to e
40、nsure exposure and counting uniformity. See, for example,Ref (11). For counting, dual detectors at 180 can be used as analternative to rotation to minimize positioning errors at thecounting station.6.4 Since16N emits high-energy gamma rays, determina-tions are less subject to effects of self-absorpt
41、ion than aredeterminations based on the use of indicator radionuclidesemitting lower energy gamma rays. Corrections for gamma-rayattenuation during counting are usually negligible, except forlarge samples as may be needed in the highest sensitivitydeterminations.6.5 The oxygen content of the transfe
42、r container (“rabbit”)must be kept as low as possible to avoid a large “blank”correction. Suggested materials that combine light weight andlow oxygen content are polypropylene and high-density poly-ethylene (molded under a nitrogen atmosphere), high purityCu, and high-purity nickel. A simple subtrac
43、tion of the countsfrom the blank vial in the absence of the sample is not adequatefor oxygen determinations below 200 g/g, since large samplesizes may be required for these high-sensitivity measurementsand gamma-ray attenuation may be important when the sampleis present (12). If the total oxygen con
44、tent of the sample is aslow as that of the container (typically about 0.5 mg of oxygen),the sample should be removed from the irradiation containerprior to counting. Statistical errors increase rapidly as truesample activities decrease, while container contamination ac-tivities remain constant. For
45、certain shapable solids, it may bepossible to use no container at all (13). This “containerless”approach provides optimum sensitivity for low-level determi-nations, but care must be taken to avoid contamination of thetransfer system.6.6 Although the discriminator is used to eliminate thesignal origi
46、nating from gamma rays of energy less than 4.5MeV, it is possible when analyzing certain materials that veryhigh matrix activities can result in multiple gammas of lowerenergy being summed, thereby generating a signal in theenergy window. This effect can be minimized by reducing thespecific activity
47、 of the interfering radionuclide or by alteringthe counting geometry to reduce the solid angle. Since thedecay of these “coincidence” events are subject to the half-lifeof the radionuclide from which they are emitted, it may bepossible to differentiate the interfering signal from oxygencounts by dec
48、ay rate if using an MCS-based sytem (14).7. Apparatus7.1 14-MeV Neutron GeneratorTypically, this is a high-voltage sealed-tube machine to accelerate both deuterium andtritium ions onto a target to produce 14-MeV neutrons bythe3H(d,n)4He reaction. In the older “pumped” drift-tubeaccelerators, and als
49、o in some of the newer sealed-tube neutrongenerators, deuterium ions are accelerated into copper targetscontaining a deposit of titanium into which tritium is absorbed.Detailed descriptions of both sealed-tube and drift-tube ma-chines have been published (1, 3).7.1.1 Other nuclear reactions may be used, but the neutronenergy must exceed 10.22 MeV (15) for the16O(n,p)16Nreaction to take place. The 14-MeV neutron output of thegenerator should be 109to 1012neutrons s1, with a usablefluence rate at the sample of 107to 109neutrons cm2s1. The14-MeV fluence rate may
